Solid waste such as MSW has traditionally presented problems of disposal which have become increasingly critical in recent years as a result of not only a rapidly increasing population but the compounding difficulty of a drastic increase in per capita production of solid waste. Conventionally, MSW has been disposed of by such means as incineration and landfill. Obviously with the ever increasing concern with problems of natural resources and the dwindling supply of acreage suitable for landfill operations within a reasonable distance of population centers, both of these methods of solid waste disposal are becoming less acceptable.
The primary strategy for disposing of MSW has been to dump it on land. In 1986 the Environmental Protection Agency (EPA) determined that more than three-fourths of all MSW was deposited in the nation's 6,000 municipal landfills. The tradition of land disposal, however, is becoming increasingly less desirable. Communities near present and proposed landfill sites have always been concerned about the reduction of property values and the smell and sight of garbage; now they also worry about physical harm to themselves from landfill gases, micro-organisms and toxins.
In response, many state environmental agencies and the EPA have legislated or proposed regulations that greatly increase standards of landfill design (and thus cost) and performance to protect people and the environment from pollutants. These regulations have closed or will soon close many landfill sites and have limited the construction of new sites.
In addition to environmental regulation, increasing costs make disposing MSW in landfills less desirable. For example, the city of Philadelphia is paying to have trash hauled as far as Harrisburg and even South Carolina at a cost $50 per ton. Increasing costs provide incentive to use alternative technologies. Most alternative solutions involve some form of burning or incineration. Traditional incineration is not an ideal solution because it is generally more expensive than land disposal and because as a consequence of the inherent nonuniformity of normal garbage, combustion is erratic such as to foster toxic fume releases.
Prodded by these incentives, attention has focused on converting MSW to a fuel, commonly referred to as Refuse Derived Fuel (RDF). Based on (Spring 1973) figures from the National Center for Resource Recovery, MSW contains a total of about 50% organic matter, in accordance with the following table:
______________________________________ COMPOSITION OF MUNICIPAL SOLID WASTE COMPONENT DRY BASIS ______________________________________ Glass 9.0% Ferrous Metal 7.0% Aluminum 0.7% Other Nonferrous 0.3% Paper 32.0% Textiles 1.8% Rubber 1.0% Plastics 1.5% Other Organic 13.7% Other Inorganic 8.0% Water 25.0% TOTAL 100.0% ______________________________________
The organic fraction of MSW is an important source of energy, which has an average heat value of about 8500 BTU per dry pound and an annual potential for the United States equivalent to nine billion gallons of No. 2 fuel oil. On a dry basis, its heat equivalent is about two-thirds that of Ohio coal, while its sulphur content is nil compared to low sulphur coal. Most important, it is a renewable source of energy which is readily available on a year-round basis in energy dependent urban areas.
In order to realize the potential of RDF, it must have sufficiently uniform physical and combustion characteristics to suit the specific requirements of the furnace or other burner in which it is to be used. From this standpoint, it is essential that the inorganic constituents be removed as completely and efficiently as possible from the organic fraction, so that all of the organic material can be processed to fuel while the ash content is minimized. A less obvious requirement is that there must also be complete separation of organic material from the removed inorganic fraction, since any organic material retained with the inorganic, which ultimately reach land fill, constitute a putrescible nuisance and potential hazard.
The use of RDF has met with a number of difficulties. First, a problem arises by virtue of the nature of MSW, which may consist of a wide variety of diverse materials, some of which can be used as a fuel while others cannot. Furthermore, the relatively low density of collected municipal wastes makes it impractical to handle this material for burning directly, even if this were otherwise feasible. In view of this fact, it has been known to compact or otherwise treat municipal wastes in a manner calculated to increase the usability thereof as a fuel. However, no really successful method of processing MSW on a large scale has been devised, either because processing costs are prohibitive or by virtue of the fact that the resultant product does not justify the processing expense.
Current RDF operations are typically large scale (100 ton +/hr.) costing tens of millions of dollars. The successful ones have generally required specially designed combustion equipment. In addition to high capital cost, the major problems which have plagued current and past RDF operations include:
1) High ash content fuel-causing boiler slugging; PA1 2) Equipment failure, including explosions, from hazardous waste contamination; PA1 5) Poor fuel conversion from available waste; PA1 6) High residual waste; PA1 7) Poor recovery of by-products; and PA1 8) Excessive transportation cost.
3) Excessive equipment wear;
4) Nonuniform fuel burning quality, due to moisture variation;
RDF should have a moisture content of 10% or less to serve as an effective fuel. Thermal drying is more effective than mechanical drying techniques at relatively low-moisture levels, particularly with MSW of high paper content. Appreciating that mechanical extraction of water requires only 10% or less of the energy of thermal (evaporation) removal, there is a reluctance to add water during the processing of MSW to produce low-moisture RDF. As a result, many of the present commercial RDF operations are dry process, use air floatation for nonferrous inorganic (grit) separation, and usually do not provide for a separate drying operation to yield uniform moisture content.
The burning of RDF results in emissions which are passed into the atmosphere. The nature and quantity of such emissions are subject to state and federal regulations, e.g., the Clean Air Act as enforced by the Environmental Protection Agency (EPA). The Clean Air Act regulates the emission of heavy metals and NOxSOx. The burning of raw, unprocessed MSW requires the use scrubbers such as electrostatic precipitators to bring emissions within the limits set by of the Clean Air Act. Such apparatus is costly to install and to operate. The EPA imposes monetary damages on industries, e.g., electric utilities, which fail to meet the limits of the Clean Air Act. These damages are defined in terms dollars per ton of pollutants. Thus, an industry which emits so many tons of pollutants in excess of the limits, must pay a fine based on the excess tons. Conversely, those industries, which burn fuel with pollutants less than the limits, will receive credits based on the difference between the limits and the emitted tons. These credits are bought and sold on the Chicago Board of Trade. Thus, industries burning clean fuel may realize income by selling its credits, whereas polluters must buy credits to burn its relatively dirty fuels.
U.S. Pat. No. 3,506,414 of Skendrovic is an example of a process for producing RDF without the introduction of water. Skendrovic discloses a system for transforming municipal refuse and garbage into a low-grade fuel. Refuse and garbage are collected and placed in a feed hopper which distributes it through a separator-disintegrator. The separator-disintegrator reduces the top size of the refuse and garbage and simultaneously dewaters the refuse and garbage. From the separator-disintegrator, the refuse and garbage pass through a metal separator to remove ferrous metals therefrom. The refuse and garbage are then passed through compressive rollers to exert a compressive force thereon, and to squeeze water therefrom. An extruder forms the dewatered material into fuel pellets.
U.S. Pat. No. 4,049,391 of Marsh discloses the depositing of MSW into a treatment vessel with water and subjecting such a mixture to mechanical and hydraulic shear forces to produce a slurry. Such a slurry is subsequently processed by a liquid cyclone and a mix tank, before being dewatered by a screw press and a cone press. The partially dried product of the cone press is finally dried by a thermal dryer. It is apparent that most of the water removal is performed by the thermal dryer in that according to Marsh, "It is not practically feasible to dewater the slurry mechanically to a higher solids content than about 50%."